Key telephone systems were developed to permit a single telephone to have access to a number of telephone and intercom lines. They also permit the telephone to be temporarily disconnected from a telephone line on which a call is in progress and still maintain a connection with the distant party. This is typically accomplished by means of a hold resistor that simulates the electrical impedance of the speech network of the telephone and is connected in shunt with the telephone prior to the telephone being disconnected from the line. This arrangement, however, leaves the distant party with what appears to be and is indistinguishable from a dead line. Therefore, a number of arrangements have been devised for applying music or a tone signal to the held line.
Those arrangements that have been made the subject of a patent are disclosed in U.S. Pat. No. 3,246,082 issued to A. Levy on Apr. 12, 1966, U.S. Pat. No. 3,794,774 issued to R. C. Kemmerly et al. on Feb. 26, 1974, and U.S. Pat. No. 3,909,553 issued to R. A. Marshall on Sept. 30, 1975. In addition, co-pending patent application Ser. No. 708,858, filed July 28, 1976, now U.S. Pat. No. 4,056,695, in the name of R. J. Angner and J. V. Lacy is directed to such an arrangement. In each of these arrangements in order to provide isolation between the audio signal source and the telephone line, the audio signal is inductively coupled to the telephone line when the line is in the hold condition.
Inductive coupling, however, introduces the possibility of cross talk between lines that are in the hold condition at the same time. This is because, as shown in the above-mentioned Kemmerly et al. patent, for reasons of economy it is necessary to use the same audio source for a multiplicity of telephone lines, the audio source being inductively coupled to each of these lines. Thus, when more than one line is on hold at the same time, a talking path is provided between the parties that are on hold.